DESCRIPTION: This application is aimed at generating further understanding of how HIV co-receptors function during viral entry and env-mediated membrane fusion. The first specific aim involves mutagenesis of the extracellular regions of CCR5, including the identification of mutants with intermediate efficiency. These will be used in the second specific aim to select for HIV-1 variants with enhanced ability to use the partially defective receptor; the sequence changes in HIV-1 involved in this adaptation will be identified, as they could contribute to an understanding of the HIV-receptor interaction. The third specific aim is to create and use CCR5-CCR3 chimeras and use these to identify env domains important for specific co-receptor useage. The final specific aim focusses on the promoter regions of CCR5, the identification of transcription factors that may interact with the promoter, and the use of antisense strategies to block CCR5 expression.